THE 



AMERICAN NATURALIST 



Vol. XL February, 1906 Xo. 470 



THE UNITY OF THE GXATHOSTOME TYPE 



HOWARD AYERS 



Since zoologists have recognized the simple nature of the 

 Cyclostome fishes there have been many contributions to our 

 knowledge of their structure and many discussions as to their 



and the clearing up of its (Icvclopincnt by tlie investigations of 

 Kowalevsky, Hatschek, and Willey, have increased the amount of 

 interest in these discussions and have added to the subject an en- 

 tirely new phase in that the amphioxus, instead of being longer 

 considered a zoological curiosity, a degenerated or aberrant form, 

 has become the center of an intense and searching discussion of 

 the origin and relationships of the Yertebrata; and amphioxus 

 has thus come into its own by being recognized as an ancestral 

 form in the genealog}^ of the vertebrate stock and the oldest living 

 relative and representative of this group of animals. 



We can now see clearly enough that the Mar>ip(.bran( hia and 

 the Acrania both stand in the relation of anct-^tors to tlic Acrte- 

 brates aBove them, and there is no longer any il(iui)T. while ncog- 

 nizing to the full the many unsolved problems in connei tion with 

 its structure and development, that amphioxus bcloni^s to the 

 group of forms, the Prospondylia, prechn essors of tlie Archicrania, 

 from which the Cyclostomes are directly descended. It must 



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